DFM Guide | Express Fab USA

Because Express Fab USA operates on a Direct-Data philosophy, the quality of your physical board is a direct reflection of your digital layout. For 0402 designs, precision is key. Follow these guidelines to eliminate common SMT failures like tombstoning and solder bridging.

1Thermal Symmetry

The primary cause of 0402 "tombstoning" is an imbalance in heat during reflow. If one pad reaches liquidus temperature before the other, surface tension pulls the component upright.

Balanced Traces: Ensure traces entering both pads are the same width.
Thermal Relief: Avoid connecting one pad to a massive copper plane while the other is on a thin trace. Use thermal spokes to balance the thermal mass.

[Diagram: Balanced vs. Unbalanced Pad Entry]

2Solder Mask & Clearance

With footprints this small, the solder mask is your primary defense against bridging.

The Mask Dam

Maintain a minimum of 4 mil (0.1mm) of solder mask between pads to prevent solder flow-together.

NSMD Recommendation

We recommend Non-Solder Mask Defined (NSMD) pads for 0402s to provide superior registration.

3Data-Based Orientation

Our system builds according to your XY (Centroid) data, not silkscreen "hints."

The Zero-Degree Rule

Ensure your EDA tool exports rotations according to IPC-7351B. Verify that your CAD library's "Pin 1" matches the physical component's cathode/anode marker.

Pre-Upload Checklist

Trace/Space is 6/6 mil or larger.

Minimum 10 mil clearance between bodies.

All 0402s share consistent MPNs in BOM.

Silkscreen is clear of all SMD pads.

Correcting these in CAD takes minutes; correcting them on the shop floor takes days. Design for success.

0402 Design for Manufacturing (DFM)